Ionizing radiation may include high speed, energetic, subatomic particles, ions or small atoms, or the like. Without wishing to be bound by theory, ionizing radiation may remove electrons from atoms or molecules when it passes through or collides with a material. The ionized atoms or molecules can undergo radiolysis and form free radicals to trigger further chemical reactions. One form of ionizing radiation is an electron beam or e-beam radiation. E-beam radiation can be high (5 to 10 MeV), medium (500 keV to 5 MeV) or low (80 to 500 keV) in energy level.
It may be desirable to modify properties of various polymeric materials and improve mechanical, thermal, and/or chemical properties of a polymer and extend the range of applications of a polymer. Furthermore, the level of the e-beam dosage may be important in the modification and improvement of the mechanical, thermal and/or chemical performance properties of polymers such as polyolefins, which are commonly used in microporous polyolefin battery separator membranes for rechargeable batteries, such as various lithium batteries, such as lithium metal and/or lithium ion batteries.
Polyolefins, such as polyethylene (PE) and polypropylene (PP), are semi-crystalline polymeric materials which are commonly used in the manufacture of microporous separator membranes for use as battery separators in rechargeable lithium batteries. Without wishing to be bound by theory, e-beam radiation of polyolefinic materials may break C—C (4.25 eV) and C—H (3.60 eV) bonds forming free radicals which may trigger a competing process of chain scission vs. cross-linking. The predominance of chain scission vs. cross-linking is determined by a polymer's molecular weight, tacticity, and crystallinity and also by e-beam processing conditions such as pressure, temperature, inert atmosphere, and e-beam dosage.
FIG. 1 depicts a semi-crystalline polymer composed of amorphous regions and crystalline regions where polymer chains in the amorphous regions appear as loosely coiled chains tying together lamellae stacks of crystalline regions of the polymer. Chain scission predominates in the crystalline regions due to a lack of mobility of free radicals, while cross-linking prevails in amorphous regions because of polymer chain entanglements. Cross-linked polymer chains are shown in both FIG. 1 and FIG. 2. Due to the increased molecular weight and 3D network generated by cross-linking, a cross-linked polymeric material tends to have higher viscosity and mechanical strength at elevated temperature.
Polymeric materials can be used in microporous battery separator membranes. A battery separator may include a microporous membrane placed between the cathode and anode in a battery system in order to prevent physical contact between the anode and cathode while allowing electrolytic ionic flow during charging and discharging cycles in a battery. There is a growing demand for high energy density, secondary lithium batteries (for example, in some instances, lithium ion batteries).
There is a growing demand for high energy lithium batteries for consumer electronics applications, such as smart phones and laptop computers, for power tools and for electric/hybrid electric vehicle applications. Some such rechargeable or secondary lithium batteries include lithium ion batteries. Lithium ion batteries may include a high performance microporous separator membrane.